Since their discovery in the 1930's, urethane foams have been used in a wide variety of applications. One example of such applications include their use as media for the filtration of gases and liquids. To be useful as filtration media, however, as well as for certain other applications, such foams must possess a relatively high level of permeability. As many urethane foams inherently possess a lower level of permeability than is desirable for such applications, those in the art have developed various methods to address this issue. These methods may be conveniently divided into two groups in accordance with the timing of their use, i.e., pre-formation methods and post-formation methods.
Post-formation methods of foam permeability enhancement are those methods which are used in connection with foams that have been fully-formed and cured. One well-known post-formation method is explosion reticulation. This method is suitable for use in regard to both polyester and polyether urethane foams, i.e., foams prepared by reacting isocyanate with a polyester polyol or a polyether polyol, respectively. A second such method, which has been typically used only in connection with polyester urethane foam, comprises immersing the fully-formed foam into a caustic bath which dissolves a certain portion of the foam, thereby increasing the permeability of the foam.
As the foregoing post formation processes can be used only after the foam is fully-formed those in the art turned to developing more efficient means of preparing highly permeable urethane foams. Eventually, methods were developed which provided such foams without requiring post-formation processing. These methods, which can be referred to as pre-formation methods, add certain components into specifically formulated foam-forming reaction compositions. Those components assist in generating the desired level of permeability in the resulting foam during its preparation, i.e., without requiring any post-formation treatment of the foam, and are according more efficient than post-formulation methods. Examples of pre-formulation methods are provided by U.S. Pat. Nos. 3,748,288, 3,884,848, and 4,826,882.
Despite the known advantages of pre-formation methods for preparing urethane foams, those in the art have been unable to prepare highly permeable urethane foams by any pre-formation method when only polyether polyols, or only polyester polyols, are used as the polyol component.
In particular, those skilled in that art have been unable to obtain a foam product when only polyester polyols are included in compositions used in existing pre-formation methods. Specifically, compositions containing only polyester polyols will typically destabilize and boil. Even in those situations where a rising foam mass is able to be formed, it will eventually destabilize and collapse.
In contrast, those skilled in the art have been able to prepare foams using compositions which include only polyether polyols. However, such foams exhibit low permeabilities, e.g., about 4 ft.sup.3 /min and below, as well as a limited range of densities, e.g., from about 0.75 to about 4 lb/ft.sup.3. Foams having permeabilities below about 2 ft.sup.3 /min are generally considered to be unsuitable for use as filter elements in most applications.
One pre-formation method which is, in essence, a compromise solution to the problems inherent in the polyester- and polyether-only compositions described previously, is Velve.RTM. foam. (General Foam Corporation, West Hazleton, Pa). This urethane foam is prepared from a complex composition which necessarily includes, inter alia, a combination of polyester and polyether polyols. In addition to that combination of polyols, emulsifiers must also be included in the composition. Emulsifiers, which are typically included in foam-forming compositions, function to stabilize the composition as the foam-forming reaction proceeds, i.e., prevent the composition from boiling and/or collapsing. Selection of the type of emulsifier generally depends upon the type of polyol that is predominant in the composition. The Velve.RTM. foam composition is unusual in that respect because, as a composition which includes a polyester-type polyol as its predominant polyol component, it uses a silicone-based emulsifier that is typically used in compositions having polyether type polyols.
The increase in stability provided by such emulsifiers, however, also yields a less permeable foam product. To counteract the decrease in permeability attendant with the addition of an emulsifier, cell-opening agents are included in the composition. The end result is a foam-forming composition which achieves a tenuous balance between adequate permeability in the final foam product and stability of the composition during preparation.
Preparing a foam using the Velve.RTM. foam composition, however, is a very difficult procedure. In order to obtain the desired high degree of permeability, the process must be operated such that the foam composition, while rising, is continually on the verge of collapsing. Thus, in the preparation of a highly permeable foam using this known method, one is faced with the problem of controlling a very unstable and complicated process.
The method for preparing the Velve.RTM. foam product is further limited in regard to the type of foam that can be obtained. More specifically, foams prepared by this method are primarily relatively soft foams, making these foams unsuitable for certain applications which require relatively firm foams. In addition, any substantial deviation from the Velve.RTM. foam formulation, e.g., eliminating one of the polyols completely such that only a polyether polyol (or only a polyester polyol) is used, or significantly altering the quantities of the various components, typically results in either collapse of the rising foam composition or, if a foam is ultimately produced, an unusable product.
Further, due to its inclusion of primarily polyester polyols in its formulation, the Velve.RTM. foam method provides urethane foams which are useful for applications where the foam is to be exposed to organic solvents. However, this type of urethane foam would not be suitable in applications where the foam will experience high levels of heat and humidity.
Accordingly, it is an object of the present invention to provide a method for preparing a flexible, and permeable, urethane polyester foam in which the tendency for the foam to collapse during preparation is minimized, without sacrificing permeability in the final foam product.
A related object of the present invention is to provide a method for preparing a polyester urethane foam that is relatively easy to control.
Yet another object of the present invention is to provide a polyether urethane foam which possesses a high degree of permeability over a relatively wide range of densities.
These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.